The invention relates to a pin extruder for conveying polymeric material, especially elastomeric material, but also natural rubber.
Such extruders have been successfully used for many years. For example, DE-OS 22 35 784 shows a pin extruder from 1972 that is already adjustable with respect to the radial penetration depth of the pins. The pins extend to the extruder screw and are secured to the housing, whereby screw ribs of the extruder screw are provided with respective openings that enable the pertaining pin to pass through at this axial height.
Although it is often assumed that the greater throughput of such a pin extruder in comparison to an extruder without pins is based on the fact that the rotation flow is obstructed in the screw channel, more recent tests show that the throughput gain results from the increasing relative speed between the fixed pins on the one hand and the rotating screw flanks on the other hand. In practice the extruded material is constantly pushed ahead of the pins and is pushed through the openings in the conveyor ribs in a conveying effective manner and partially in the direction of the extrusion nozzles, and in particular in the portion of the extruded material that is conveyed there and is located downstream.
It is furthermore known to fix the pins on the screw and to this extent to undertake a kinematic exchange. For this purpose, reference is made for example to DE-GM 71 03 071. With this solution, which is intended for double-screw extruders, practically two differently arranged and embodied pin arrangements mesh with one another. A drawback of this is that no conveying effect results, although the mixing effect of these screws is good. On the other hand, this solution is only suitable for double-screw extruders.
A similar solution, which in principle is also suitable for single extruders, is known from DE-OS 26 50 248. With this arrangement, rows of pins extend in the manner of annular grooves that, however, are not continuous but rather are formed by a plurality of inwardly extending projections. Here also there results practically no conveying effect, but a great intermixing, as is desirable for plasticizing and homogenization of the extruded material.
A drawback of the known pin extruders is that a plurality of pins are required, so that the flow channel is greatly narrowed by the pin cross-sections. This produces a very great pressure loss, so that the advantage of the pin extruder with regard to the gain in throughput is again compensated for by the high pressure drop in the pin zone and the thus accompanying loss of throughput.
A number of other forms of pin extruders have also been proposed. Most of the solutions proposed since the beginning of the 70's and partially also realized have pins that extend radially inwardly, in other words proceeding from the extruder housing. Such pins are also partially combined with transfer-mix extruders in order to further improve the thorough mixing of the cold extruded material. However, the problem continues to be the throughput, which remains somewhat below expectations.
It is therefore an object of the present invention to provide a pin extruder that with regard to the ability to thoroughly mix is improved even with very viscous extruded material without adversely affecting the throughput.